Recording materials have been disclosed on which records are made thermally by the use of intense radiation like laser beams having a high energy density. In such thermal recording or heat mode recording materials information is recorded by creating differences in reflection and/or in transmission optical density on the recording layer. The recording layer has high optical density and absorbs radiation beams which impinge thereon. The conversion of radiation into heat brings about a local temperature rise, causing a thermal change such as evaporation or ablation to take place in the recording layer. As a result, the irradiated parts of the recording layer are totally or partially removed, and a difference in optical density is formed between the irradiated parts and the unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and 4,291,119 and British Pat. No. 2,026,346)
The recording layer of such heat mode recording materials is usually made of metals, dyes, or polymers. Recording materials like this are described in "Electron, Ion and Laser Beam Technology", by M. L. Levene et al.; The Proceedings of the Eleventh Symposium (1969); "Electronics" (Mar. 18, 1968), P. 50; "The Bell System Technical Journal", by D. Maydan, Vol. 50 (1971), P. 1761; and "Science", by C. O. Carlson, Vol. 154 (1966), P. 1550.
Recording on such thermal recording materials is usually accomplished by converting the information to be recorded into electrical time series signals and scanning the recording material with a laser beam which is modulated in accordance with the signals. This method is advantageous in that recording images can be obtained on real time (i.e. instantaneously). Recording materials of this type are called "direct read after write" (DRAW) materials. DRAW recording materials can be used as a medium for recording an imagewise modulated laser beam to produce a human readable or machine readable record. Human readable records are e.g. micro-images that can be read on enlargement and projection. An example of a machine readable DRAW recording material is the optical disc. To date for the production of optical discs tellurium and its alloys have been used most widely to form highly reflective thin metal films wherein heating with laser beam locally reduces reflectivity by pit formation (ref. e.g. the periodical `Physik in unserer Zeit`, 15. Jahrg. 1984/Nr. 5, 129-130 the article "Optische Datenspeicher" by Jochen Fricke). Tellurium is toxic and has poor archival properties because of its sensitivity to oxygen and humidity. Other metals suited for use in DRAW heat-mode recording are given in U.S. Pat. No. 4,499,178 and U.S. Pat. No. 4,388,400. To avoid the toxicity problem other relatively low melting metals such as bismuth have been introduced in the production of a heat-mode recording layer. By exposing such a recording element very shortly by pulses of a high-power laser the radiation is converted into heat on striking the bismuth layer surface. As a result the writing spot ablates or melts a small amount of the bismuth layer. On melting the layer contracts on the heated spot by surface tension thus forming small cavitations or holes. As a result light can pass through these cavitations and the density is lowered to a certain Dmin value depending on the laser energy irradiated.
Heat mode recording materials usually do not require development and fixing processes and do not require darkroom operations because of their insensitivity to room light. Therefore they constitute a valuable alternative to conventional photosensitive materials based on silver halide emulsions, e.g. for phototype-setting or image-setting applications. As is generally known silver halide materials have the advantage of high potential intrinsic sensitivity and excellent image quality. On the other hand they show the drawback of requiring several wet processing steps employing chemical ingredients which are suspect from an ecological point of view. For instance the commonly used developing agent hydroquinone is allergenic and the biodegradation of disposed phenidone is too slow. As a consequence it is undesirable that depleted solutions of this kind would be discharged into the public sewerage; they have to be collected and destroyed by combustion, a cumbersome and expensive process.
However, recording elements based on a thin metal layer show the drawback that the thin metal film may reflect more than 50% of the laser radiation, wasting the energy of the laser radiation. Accordingly, such material may require a substantial amount of energy for recording. Therefore, a high output laser light source is required if records are to be made by high-speed scanning. Methods to reduce reflectance are proposed in the Japanese Unexamined Patent Publications Nos. 40479/71 and 74632/76. However the proposed solutions have other drawbacks. Moreover, due to the high specular reflectance interference patterns arise with periods depending on the thickness of the protective cover usually present to protect the scratch-sensitive metal layer. As a consequence of these interference phenomena the finished image has an uneven and splodgy appearance.
It is an object of the present invention to provide an improved heat mode recording element based on a thin metal layer which shows reduced or no interference patterns on laser recording.
It is a further object of the present invention to provide a method for the formation of a heat mode image which has no uneven or splodgy appearance.